Downhole tools have long been used for performing various operations in wellbores. Such tools are regularly used in the oil and gas industry in various stages from drilling a well to its completion ready for production of hydrocarbons from a sub-surface reservoir.
There are a number of stages involved in forming and preparing a well for hydrocarbon production. These stages include the deployment of different types of tool or tubing strings, which typically comprise a number of tubular sections or pipe sections joined together. Various downhole tools may be mounted to or incorporated into such strings.
The first stage in obtaining hydrocarbons from a well is a drilling stage, where a drill string is used with a cutting drill bit for penetrating into the earth.
After drilling, the wellbore is usually subjected to one or more casing stages. Casing involves locating a tubular lining in the wellbore such which prevents the wellbore walls collapsing; and provides a flow path for recovery of well fluids to surface.
The casing is secured in location by cement that is set in an annular space between the casing and the wellbore wall, the cement entering the borehole from near the bottom of the casing and passing up the annular space between the wellbore wall and the casing.
It is important that the cementation in the annular space provides a seal around the casing such that fluid cannot penetrate or flow through the cement in the annular space. Accordingly, when cementing, it is important to keep the casing central in the wellbore. Centralisation may be achieved by including casing centraliser tools in the casing string, as is known in the art.
Often there are several stages of drilling and casing. For example, it is usual to drill to a certain depth, case a portion of the wellbore, drill to a further depth and then case the next portion of the well, and so on in a successive fashion until the well is drilled and cased to the required depth.
Once the drilling is completed and the wellbore cased, the wellbore is then cleaned and prepared for completion.
This is carried out by running clean-out strings, which can include combinations of tools for cleaning or polishing interior surfaces of the casing, to remove debris and junk from the well. For example, scrapers, brushes, wipers, and/or fluid jetting tools are often incorporated in a clean-out string.
After the clean-out operation, it is usual to conduct various logging operations. In these operations, properties of the wellbore are measured and logged using wellbore logging tools. These wellbore logging tools are typically suspended in the wellbore on one end of a line connected to a spool at the surface, and may be suspended on a wireline which includes direct power and data communication lines to the surface. Actual measurement is carried out on extraction of the logging tools when the line is in tension, rather than during insertion, because it gives the operator control of the tool, allows it to be extracted at a steady speed and assists in keeping the logging tools central in the wellbore.
Cement evaluation tools are used to log data concerning the quality of the cement and thus of the cement bond in the cased portions of the wellbore. It is important to run these tools to check for any defective regions of the cement, where, for example, cracks and conduits may be located providing alternative fluid paths in which hydrocarbons can flow. Such paths can cause hydrocarbons to be lost from the reservoir and can be detrimental to the control and effective recovery of hydrocarbons from the well.
Once the cement bond has been checked and is considered to be of sufficient quality, the next stage is to run a production string into the well, which provides completion equipment for extracting well fluids.
It is clear that each of these operational stages combine to give rise to significant expense associated with the drilling and completion of a well. A particular problem is that cement bond logging tools are run independently taking up large amounts of rig time. For example, a typical cement evaluation log carried out over a 100-meter casing section takes between 12 to 24 hours to complete, and can involve around ½ million dollars of rig time. Thus, cement logging performed as an independent operation is costly.
It is an object of the present invention to obviate or at least mitigate some of the drawbacks and deficiencies of existing tools and methods.
Other aims and objects of the invention will become apparent from the description below.